Fatigue testing system for prosthetic devices

ABSTRACT

A fatigue testing system provides simultaneous cycle testing for a plurality of prosthetic devices under simulated physiological loading conditions. A plurality of sample holders containing test samples of prosthetic devices is positioned between a distribution chamber and a return fluid chamber to form an integrated test chamber. A reciprocating linear drive motor operates a rolling bellows diaphragm to cyclically pressurize fluid within the test chamber and drive the pressurized fluid through the prosthetic devices being tested. The test chamber defines a return flow conduit in fluid communication with each of the sample holders, the return fluid chamber, and the distribution chamber. Compliance chambers and throttle valves associated with each of the sample holders regulate the pressure gradient and back pressure across the prosthetic devices being tested.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/137,313 filed 20 Dec. 2013 entitled “Fatigue testing system forprosthetic devices,” which is a continuation of U.S. application Ser.No. 12/718,316 filed 5 Mar. 2010 entitled “Fatigue testing system forprosthetic devices,” which claims the benefit of priority pursuant to 35U.S.C. §119(e) of U.S. provisional application No. 61/158,185 filed 6Mar. 2009 entitled “Apparatus and method for fatigue testing ofprosthetic valves,” which are hereby incorporated herein by reference intheir entirety.

TECHNICAL FIELD

The technology described herein relates to systems and methods forfatigue testing of prosthetic devices, in particular, but not limitedto, prosthetic vascular and heart valves, under simulated physiologicalloading conditions and high-cycle applications.

BACKGROUND

Prior prosthetic valve testing apparatus and methods typically use atraditional rotary motor coupled with mechanisms to produce regularsinusoidal time varying pressure field conditions. To accuratelysimulate physiologic conditions and/or produce a more desirable testcondition, especially at accelerated testing speeds, a non-sinusoidaltime dependent pressure field may be desired. This is not easilyaccomplished with a mechanistic approach. Furthermore, current systemsemploy a flexible metallic bellows or conventional piston and cylinderas drive members to provide the pressure actuation. Flexible metallicbellows are not ideal because they require high forces to operate andresonate at frequency, necessitating the use of larger driving systemsand limiting the available test speeds. Piston and cylinder arrangementsare not ideal because the seals employed in these systems are subjectedto fiction and thus have severely limited life in high cycleapplications.

The information included in this Background section of thespecification, including any references cited herein and any descriptionor discussion thereof, is included for technical reference purposes onlyand is not to be regarded subject matter by which the scope of theinvention is to be bound.

SUMMARY

A design for a fatigue testing system for cyclic, long-term testing ofvarious types of prosthetic devices (e.g., cardiac valves, vascularvalves, stents, atrial septal defect technologies, vascular linings, andothers) is designed to impart a repeating loading condition for the testsamples during a test run. However, the system is also designed to bevariable in its abilities so it can accurately test multipletechnologies. Thus, the system may be variably configured depending uponthe device being tested to impart a particular loading profile torepeatedly expose the prosthetic device being tested to desiredphysiological loading conditions during a testing run. The purpose is tosimulate typical or specific physiologic loading conditions on avascular or heart prosthetic valve, or other prosthetic technology, ataccelerated frequency over time to determine the efficacy, resiliency,and wear of the devices.

Fatigue testing is accomplished by first deploying the prosthetic devicein an appropriately sized sample holder, e.g., a rigid or flexible tube,canister, housing or other appropriate structure for holding the devicebeing tested. The sample holder is then placed between two halves of atest chamber that together form a reservoir for a working fluid. Thetest chamber is in turn mounted to a drive system. The sample holder andvalve being tested are then subjected to physiological appropriateconditions which may include: pressure, temperature, flow rate, andcycle times.

An implementation of a drive system for the fatigue testing system mayinclude a linear actuator or magnetic-based drive motor coupled to aflexible rolling diaphragm. The drive system is coupled to a loweropening in the test chamber and is in fluid communication with the fluidreservoir in the test chamber. The flexible rolling diaphragm (or“rolling bellow”) is reciprocally moveable to pressurize anddepressurize fluid and interacts with the lower section of the fluidreservoir to provide a motive force to drive the working fluid throughits cycles within the test chamber, including the sample holders.

Testing and test conditions are controlled by a control computer thatpermits both input of test conditions and monitors feedback of the testconditions during a testing run. Computer system control may be eitheran open loop control that requires user intervention in the event acondition falls outside pre-set condition parameters or a closed loopcontrol system in which the computer monitors and actively controlstesting parameters to ensure that the test conditions remain within thepre-set condition parameters.

The fatigue tester is capable of simulating physiologic conditions onprosthetic devices at an accelerated rate. The fatigue tester may alsobe configured to create either sinusoidal or non-sinusoidal pressureand/or flow waveforms across the prosthetic devices. Pressure waveformsmay also be applied that produce a pre-defined pressure gradient overtime to a prosthetic device being tested.

In one exemplary implementation, a device for simultaneous cyclictesting of a plurality of prosthetic devices is composed of a testchamber, a drive motor and a fluid displacement member. The test chamberis pressurizable and has a fluid distribution chamber with a firstmanifold defining a plurality of ports configured to receive andfluidicly couple with a first end of each of a respective plurality ofsample holders. The fluid distribution chamber also defines an aperturein a lower face in fluid communication with a pressure source. The testchamber also has a fluid return chamber with a second manifold disposedopposite and spaced apart from the first manifold of the fluiddistribution chamber. The manifold of the return chamber defines aplurality of ports configured to receive and fluidicly couple with asecond end of each of the respective plurality of sample holders. Afluid return conduit both structurally and fluidily connects the fluiddistribution chamber to the fluid return chamber. The test chamber alsohas a compliance chamber which provides a volume for holding a gas or anelastic material that compresses under a pressure placed upon fluid inthe test chamber and allows fluid in the test chamber to occupy aportion of the volume. The drive motor is configured to operatecyclically, acyclically, or a combination of both. The fluiddisplacement member is connected with and driven by the drive motor toprovide the pressure source that increases and decreases a pressure onfluid in the test chamber. In this manner, cyclic and acyclic fluidpressures may be maintained throughout the test chamber.

In another exemplary implementation, a device for accelerated cyclictesting of a valved prosthetic device includes a pressurizable testchamber. The pressurizable test chamber contains test system fluid andis further composed of a fluid distribution chamber, a fluid returnchamber, a fluid return conduit, and an excess volume area. The fluiddistribution chamber is positioned on a first side of the valvedprosthetic device and is in fluid communication with a pressure source.The fluid return chamber is positioned on a second side of the valvedprosthetic device. The fluid return conduit is both structurally andfluidily connects the fluid distribution chamber to the fluid returnchamber. The excess volume area is in fluid communication with the fluidreturn chamber and provides a volume for storing a volume of a testsystem fluid when the test system fluid is under compression.

In a further exemplary implementation, a method is presented foroperating an accelerated cyclic test system for evaluating a valvedprosthetic device. A volume of test system fluid is stored in an excessvolume area during a system driving stroke that opens the valvedprosthetic device. The stored volume of test system fluid is thenreleased during a return stroke that closes the valved prostheticdevice.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. A moreextensive presentation of features, details, utilities, and advantagesof the present invention is provided in the following writtendescription of various embodiments of the invention, illustrated in theaccompanying drawings, and defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a combined isometric view and schematic diagram of anexemplary implementation of a fatigue testing system and a correspondingcontrol system.

FIG. 2A is a front elevation view of the fatigue testing system of FIG.1.

FIG. 2B is an enlarged view of the motor support in the area surroundedby the circle labeled 2B in FIG. 2A.

FIG. 3 is a partial cross sectional view of a test chamber of thefatigue testing system taken along line 3-3 of FIG. 1.

FIG. 4A is an isometric view in cross section of a portion of thefatigue testing system if FIG. 1 detailing a flexible rolling diaphragmpump connected to a linear piston drive system in a down strokeposition.

FIG. 4B is an isometric view in cross section of a portion of thefatigue testing system if FIG. 1 detailing a flexible rolling diaphragmpump connected to a linear piston drive system in an upstroke position.

FIG. 5A is an isometric view in cross section of a portion of the testchamber of the fatigue testing system detailing an isolation valve in anopen position.

FIG. 5B is an isometric view in cross section of a portion of the testchamber of the fatigue testing system detailing the isolation valve in aclosed position.

FIG. 6 is a schematic diagram in cross section of an alternativeimplementation of a test chamber for use in a fatigue testing system.

FIG. 7 is a graph depicting three exemplary pressure control waves forgeneration by test control software to provide pressure across a sampledevice being tested.

FIG. 8 is a schematic diagram of a software and hardware implementationfor controlling a fatigue testing system.

FIG. 9 is a schematic diagram of an exemplary computer system forcontrolling a fatigue testing system.

DETAILED DESCRIPTION

The system of the present invention generally includes a linear actuatoror magnetic based drive coupled to a flexible rolling bellows diaphragmto provide variable pressure gradients across test samples mounted in atest chamber housing a fluid reservoir. These components operatetogether to act as a fluid pump and, when combined with a fluid controlsystem, provide the absolute pressure and/or differential pressure andflow conditions necessary to cycle test prosthetic devices mounted inthe test chamber. The flexible rolling diaphragm thrusts toward thelower section of the test chamber to provide a motive force to drive theworking fluid through cycles within the test chamber. The flexiblediaphragm is coupled to a lower opening in the test chamber and isreciprocally moveable to pressurize and depressurize fluid within thelower section of the main housing. The flexible rolling bellowsdiaphragm drive system has a very low inertia as compared to other drivesystems, e.g., a metal bellows or a standard piston-in-cylinder drive.The flexible diaphragm is highly compliant with low resistance to axialdeformation across its entire axial range of motion.

Plural sample holder tubes are coupled in parallel across the testchamber which has plural fluid distribution channels in communicationwith each of the sample holders. The lower distribution chamber of thetest chamber has a single fluid reservoir in fluid flow communicationwith each of the plurality of sample holders. The distribution chamberincludes a manifold with a plurality of fluid outlet ports, and eachfluid outlet port communicates with an inflow opening of a test holder.The upper return chamber of the test chamber includes a similar manifoldwith a plurality of fluid inflow ports and compliance chambers. Eachfluid inflow port communicates with an outflow opening of a respectivesample holder. A central return flow channel is provided between thereturn chamber and the distribution chamber of the test chamberreservoir to provide a return flow of the working fluid from the outflowsection of the sample holders. A throttle control and a check valve aredisposed at the inflow and outflow ends of the central return flowchannel, respectively, to regulate fluid flow during testing. Thethrottle control serves to partially regulate the pressure across theprosthetic devices being tested as well as the return flow of theworking fluid in the fluid test chamber.

These components operate together to provide a differential pressure andflow conditions necessary to cycle the prosthetic device. The internalconditions, which may include, among other things, temperature,differential pressure, and system pressure, are electricallycommunicated to monitoring and controlling software on a test systemcomputer. The motion of the fluid pump and therefore the system dynamicsare controlled via test system control software. The pressure fieldresulting from the pump motion is easily controlled and can be set as asimple sine wave or as any complex user created waveform.

One exemplary implementation of a fatigue testing system 100 for any ofa variety of prosthetic devices is depicted in FIGS. 1 through 5B. Thefatigue testing system 100 may be understood as having two primarycomponents, a test chamber 106 and a drive motor 105. The fatiguetesting system 100 may be both partially mounted upon and housed withina base housing 157 formed and supported by a number of frame members102. In the implementation shown, the drive motor 105 is housed withinthe base housing 157 while the test chamber 106 is supported on a baseplate 111 forming a top surface of the base housing 157. The basehousing 157 may be open on its sides or enclosed with removable panelsas desired. A number of leveling feet 101 may be attached to the bottomof the base housing 157 to provide for leveling of the fatigue testingsystem 100 and thereby assist in creating consistent operatingconditions across each of the multiple sample chambers of the fatiguetesting system 100 as further described below.

In the embodiment shown in FIGS. 1 and 2A with greater detail shown inFIG. 2B, the drive motor 105 may be a linear motor. The linear drivemotor 105 is mounted vertically on a motor stabilizer 104 and iscentered underneath the test chamber 106. The lower end of the drivemotor 105 is supported by a spring 103 that interfaces with a horizontalfoot 104 a of the motor stabilizer 104 that extends underneath thelinear drive motor 105. A lower shaft extension 156 is attached to andextends downwardly from a thrust rod 108 of the linear drive motor 105,through the spring 103, and through a bearing-lined aperture in the foot104 a of the motor stabilizer 104 where it is secured on the undersideof the foot 104 a as shown in FIG. 2B. The spring 103 is mountedconcentrically about the lower shaft extension 156 and abuts the lowerend of the thrust rod 108 of the linear motor 105 to support the weightof the thrust rod 108 of the linear drive motor 105 when the lineardrive motor 105 is in an unpowered state. Further, in the event of apower failure during operation, the spring 103 prevents the thrust rod108 from dropping quickly and causing an extremely large pressuregradient across test samples in the test chamber 106, which could damagethe test samples.

The upper end of the linear drive motor 105 is fixed to a motor supportplate 109 that is in turn supported by several frame members 102 of thebase housing 157. An aperture in the motor support plate 109 providesfor passage of the upper end of the thrust rod 108 for alignment andmechanical connection with components interfacing with the test chamber106. It should be noted that while a linear drive motor 105 is depictedin the figures, other types of motors, for example, a regular DC motoror a voice coil, may be used to offer alternate functionality than thelinear drive motor 105 of desired for a particular prosthetic device.The number of cycles completed during a test session may be monitored bya cycle counter (not shown) that monitors the motion of the thrust rod108 of the linear motor 105, which may be affixed to the motor supportplate 109.

Several linkage components are provided between the drive motor 105 andthe test chamber 106 as shown to best advantage in FIGS. 2A and 3. Thesecomponents together form the drive member for creating a drivingpressure on fluid in the system. An upper shaft extension 112 is fixedto and extends from an upper end of the thrust rod 108 and furtherextends axially within a cylinder 113. The cylinder 113 is supportedabout the upper shaft extension 112 within an alignment collar 107. Thealignment collar 107 is further supported by a pair of alignment mounts155 that are fixed to and extend upward from the motor support plate109. The alignment collar 107 ensures that the cylinder 113 and thedrive motor 105 maintain axial alignment.

A central aperture in the bottom of the cylinder 113 receives the uppershaft extension 112. This aperture in the bottom wall of cylinder 113 islined with a bearing 162 to provide a low friction interface between thecylinder 113 and the upper shaft extension 112 as the upper shaftextension 112 moves within the cylinder 113 as further described below.A piston 114 in the shape of an inverted cylindrical cup may be mountedto the top of the upper shaft extension 112 for axial displacementwithin the cylinder 113 as the linear drive motor 105 drives the uppershaft extension 112 up and down.

The fluid drive member can be sized based on the volumetric requirementsof the test chamber 106 or test samples. Depending on the volumedisplacement required for a particular application, the cylinder 113 andcorresponding piston 114 may be swapped out for sizes of larger orsmaller diameter. The cylinder 113 and corresponding piston 114 may thusbe provided in a variety of diameters in order to provide differentvolume displacements for a given stroke, and thereby pressuredifferentials, within the test chamber 106 depending upon the type ofsample being tested or the particular testing protocol beingimplemented.

The top edge of the cylinder 113 extends radially to form a flange 113 athat interfaces with a bottom surface of a drive adapter 117. The driveadapter 117 may also be provided in a variety of alternative differentsizes to interface with different types of cylinders 113 of variousdiameters. Thus, corresponding drive adapters 117 and alignment collars107 are similarly swapped for adjustment to the size of the cylinder113. For example, as shown in FIG. 3, the aperture in the drive adapter117 is shaped as a frustum with a wider upper mouth toward the testchamber 106 and walls that then taper to a smaller opening at theinterface with the top of the cylinder 113. In another embodiment with alarger diameter cylinder 113, the drive adapter 117 may have a largerdiameter lower opening defining more sharply angled sidewalls of thefrustum-shaped aperture and, in some embodiments, might even becylindrical to accommodate a larger cylinder 113 of a common diameter.

In one implementation, the fluid drive member has a flexible diaphragmdrive system as illustrated in FIGS. 3, 4A, and 4B. The diaphragm 115may be a cap-like or cup-like member constructed of a non-reactive andflexible thin rubber, polymeric or synthetic based material. Theflexible diaphragm 115 is highly compliant with low resistance to axialdeformation across its entire axial range of motion within the cylinder113 and the aperture in the drive adapter 117. However, alternativeconfigurations of the diaphragm 115 may be employed so long as theconfiguration is capable of low friction and low resistance todeformation under the influence of the piston 114. The outer diameter ofthe sidewalls 114 a of the piston 114 is slightly smaller than the innerdiameter of the cylinder 113 to provide a uniform gap therebetween. Thegap is designed to be large enough to allow the sidewall of thediaphragm 115 to fold in half against itself, i.e., evert, within thisgap when the sidewalls 114 a of the piston 114 are positioned within thesidewalls of the cylinder 113. It may thus be noted that in operationflexible diaphragm 115 will operate as a rolling bellows when the lineardrive motor 105 is actuated.

Many advantages of a low friction flexible diaphragm 115 as opposed to arigid metallic bellows or traditional piston and cylinder drive may beappreciated. The lateral surfaces of the diaphragm 115 evert between thesidewalls 115 a of the piston 115 and the sidewalls of the cylinder 113as the piston 114 reciprocates within the cylinder 113 and drive adapter117. However, this eversion occurs with very low friction and low heatgeneration, and exerts very little resistance to piston 114 movement. Assuch, the drive motor can operate with a low driving force for eithersmall or large displacements requiring low current draw and thus lowenergy expenditure. The flexible diaphragm 115 is highly compliant withlow resistance to axial deformation across its entire axial range ofmotion. Alternative configurations of the diaphragm 115 may also beemployed so long as the configuration is capable of very low fiction andvery low resistance to deformation under the influence of the piston114.

The cup-shaped flexible diaphragm 115 is mounted on top of the piston114. The end wall of the diaphragm 115 is held against the top of thepiston 114 by a rigid, disk-shaped cap 116, which is fastened to theupper shaft extension 112 via fastener 158 (e.g., a set screw threadedwithin the upper extension shaft 112). A flange surface 115 a extendsradially outward and circumferentially around the opening to the cavityof the cup-shaped diaphragm 115. This flange surface 115 a is sandwichedbetween the flange surface 113 a of the cylinder 113 and the bottomsurface of the drive adapter 117 to maintain a pressure seal between thetest chamber 106 and the drive components. In view of FIG. 3, thetapering of the frustum-shaped wall defining the aperture in the driveadapter 117 to an opening of comparable diameter to the diameter of thesidewalls forming the cylinder 113 becomes apparent, i.e., the need forcorresponding surfaces between the flange of the cylinder 113 and thebottom surface of the drive adapter 117 to retain the flange surface 115a of the diaphragm 115.

A top surface of the drive adapter 117 is fastened to a plenum 118 thatdefines a center cylindrical aperture aligned coaxially with theaperture of the drive adapter 117 and the cavity defined by the cylinder113. As shown in FIGS. 4A and 4B, a plurality of ports may be definedwithin the sidewall of the plenum 118 to provide fluid fill, drainage,or other functionality within the linkage components below the testchamber 106. For the sake of clarity, the plenum 118 shown in FIG. 3depicts only a single sidewall port 143 that, in this embodiment, isused as a fluid inflow port. However, additional ports may be definedwithin the plenum 118 if desirable (see, e.g., FIGS. 4A and 4B). If notin use for a particular test configuration, the sidewall ports 143 inthe plenum 118 may be plugged. The plenum 118 may be of constantdiameter to interface with an aperture in the base plate 111 of the basehousing 157. For this reason it can now be understood why in someembodiments the opening in the top surface of the drive adapter 117 maybe of a larger diameter than the opening in the bottom surface of thedrive adapter 117 as the drive adapter 117 mates with a constantdiameter plenum 118 in contrast to a variable diameter cylinder 113.

As previously indicated, the test chamber 106 sits atop of the baseplate 111 on the base housing 157. The foundation of the test chamber106 may in some embodiments include a gate guide plate 120 that alsodefines a central aperture that is coextensive in diameter with andconcentrically aligned with the aperture in the base plate 111. Adistribution chamber 126 is supported by the gate guide plate 120. Thegate guide plate 120, the distribution chamber 126, and the base plate111 may all be fastened together via bolts (not shown) extending upwardfrom the underside of the base plate 111. A pair of gate seals 160 maybe positioned between the gate guide plate 120 and the distributionchamber 126. The gate seals 160 may be a set of stacked O-ringssurrounding the central apertures in the gate guide plate 120 and thedistribution chamber 126. The gate seals 160 may be slightly recessedwithin corresponding annular grooves formed in a bottom surface of thedistribution chamber 126 and a top surface of the gate guide plate 120.

A shallow flat channel 159 of a width at least slightly greater than thediameter of the gate seals 160 may be formed to extend radially from thecenter aperture within the top surface of the gate guide plate 120. Aflat rectangular knife or gate valve 119, shown to best advantage inFIGS. 5A and 5B, may be positioned within the channel in the gate guideplate 120. The gate valve 119 is shown in a radially withdrawn, openposition in FIG. 5A, such that there is an open passage between thecenter apertures defined within the gate plate 120 and the distributionchamber 126. In this position, the gate seals 160 press against eachother to create a fluid tight seal about the central apertures.

In a closed state as depicted in FIG. 5B, the gate valve 119 is pushedradially inward such that the flat plate forming the gate valve 119extends completely across the center apertures and is sealed between thegate seals 160 to fluidically separate the gate guide plate 120 and thelinkage elements below it from the rest of the test chamber 106 aboveit. Closure of the gate valve 119 thus allows for easy maintenance ofthe drive motor 105 or the piston driver 114 (e.g., replacement of theflexible bellows diaphragm 115) without having to drain the test chamber106.

As previously noted, the distribution chamber 126 is positioned on topof the gate guide plate 120. The distribution chamber 126 defines a muchwider but shallow cavity with an interior bottom wall that slantsradially inward until it reaches a common diameter with the aperture ofthe gate plate 120. In this embodiment, one or more resistive heaters130 may be embedded within the bulk of the distribution chamber 126underneath the fluid cavity defined by distribution chamber 126. In oneembodiment a separate heater 138 may be located immediately beneath eachsample holder 129 (as further described below) to aid in the uniformityof heat distribution within the fluid in the test chamber 106.

A distribution chamber manifold 153 is mounted to a top surface of thedistribution chamber 126, for example, by bolting the two surfacestogether about the perimeter. The distribution chamber manifold 153 mayform a conical diverter surface 122 that extends downward into thecavity of the distribution chamber 126. A center bore 164 extendsthrough the distribution chamber manifold 153, including through thediverter 122. This center bore forms part of a return flow path 128 asfurther described below. The distribution chamber manifold 153 mayfurther define a plurality of bores 166 spaced about the perimeter ofthe distribution chamber manifold 153. In the embodiment shown in FIGS.1 through 3, eight perimeter bores 166 are defined at a uniform radialdistance from the center and are equiangularly spaced apart. However, inalternative embodiments, there may be greater or fewer perimeter bores166 and different or non-uniform spacing may be implemented if desired.A sample inlet tube 147 forming the bottom half of a sample holder 129is connected within and seals against each of the perimeter bores 166 ofthe distribution chamber manifold 153.

Similarly, a lower conduit wall 124(2) of a telescoping center conduit124 mounts within and seals against the center bore 164 in thedistribution chamber manifold 153. A one-way valve 127 or similar checkvalve is positioned within the center bore 164 of the distributionchamber manifold 153 below the lower conduit wall 124(2) of the centerconduit 124. The second half of the center conduit 124 is formed as acylindrical upper conduit wall 124(1) with an inner diametersubstantially the same as an outer diameter of the lower conduit wall124(2) such that the center conduit 124 can be expanded or contracted inlength. One or more center seals 161, for example, in the form ofO-rings mounted within annular recesses in the outer surface of thelower conduit wall 121(2) provide a fluid tight seal interface with theinterior surface of the upper conduit wall 124(1).

Each of the sample holders 129 is also composed of a sample outlet tube148 that sits atop a respective sample inlet tube 147. The actual testsample 130 (e.g., a prosthetic device) may be sandwiched between thesample inlet tube 147 and sample outlet tube 148 to hold it in placewithin the test chamber 106. In other embodiments (not shown) each ofthe sample holders 129 may be an integral unit with a test sample 130mounted within by any of a variety of means that may be specific to thenature of the particular test sample 130.

A return chamber manifold 154 is positioned atop each of the sampleoutlet tubes 148 and the upper conduit wall 124(1) of the center conduit124. The return chamber manifold 154 defines corresponding perimeterbores 167 for mating with each of the sample outlet tubes 148 and acenter aperture 165 for mating with the upper conduit wall 124(1) of thecenter conduit 124. Each of the interfaces between the sample outlettubes 148 and the center conduit 124 with the return chamber manifold154 is configured to provide a fluid tight seal between the components.

A return chamber 136 is mounted on top of the return chamber manifold154 in a similar manner as the distribution chamber 126 is mounted tothe distribution chamber manifold 153. The return chamber 136 definesone or more compliance chambers 135 which, in the embodiment of FIGS.1-3, are composed of cavities associated respectively with each of thesample holders 129. As used herein, “compliance” refers to the abilityof the cavities forming the compliance chambers 135 to absorb some ofthe pressure placed upon the fluid in the test chamber 106 and furtherto control recoil toward the original volume dimensions upon removal ofthe compressive force. The compliance chambers 135 assist in minimizingthe effects of large and quickly changing pressure gradients across testsamples 130 placed within the test chamber 106. In some implementations,the compliance chambers 135 may merely contain air or another gas. Theair or other gas may be indirect contact with the working fluid in thetest chamber 106 or a membrane may be provided within the compliancechambers 135 to separate the air or gas from the working fluid. In otherembodiments, the compliance chambers 135 may house a porous material oran elastomeric material. In each case, the purpose of the compliancechambers is to act as a resilient spring force to dampen the effects oflarge, quickly changing pressure gradients within the test chamber 106.

The return chamber 136 further defines separate return conduits 168 thatare in fluid communication with respective pairs of the sample holders129 and compliance chambers 135. A respective throttle valve 132 ishoused within the return chamber 136 and placed between each respectivereturn conduit 168 and a central return basin 133 that empties into thecenter return conduit 124. Each of the throttle valves 132 may beoperably raised or lowered to restrict or enlarge the fluid flow passagebetween the return conduits 168 and the central return basin 133 andassist in the regulation of the pressure across the test samples 130 inthe sample holders 129.

The return chamber 136 may also define a number of bores variously incommunication with the return conduits 168, the compliance chambers 135,the central return basin 133. For example, system input bores 121 may beprovided for relatively direct access to each of the sample holders 129via the return conduits 168. The system input bores 121 may be used fora variety of purposes, for example, for the placement of sensing ordiagnostic equipment or for access to the test samples 130 in the sampleholders 129. In a particular example, when testing a valve design withremovable valve leaflets, the system input 121 could be used to accessthe sample holder 129 and remove and replace the valve leaflets whileleaving the permanent valve mounting structure in place within thesample holder 129. This obviates the need to disassemble the testchamber 106 for replacement of components of the test samples 130.

An access port 125 may also be associated with each of the compliancechambers 135. The access port 125 may be used for the attachment andintroduction of sensing equipment or for providing additional air or gaspressurization to the compliance chambers 135. Additional sensors mayfurther be placed in access ports positioned for communication with thecentral return basin 133. For example, as shown in FIG. 3, a temperaturetransducer 139 and a water level sensor 144 may be placed incommunication with the central return basin 133. It should be understoodthat any number of other sensor devices could also be placed in contactwith or introduced within the test chamber 106 by any of the variousapertures, bores, or ports provided within the return chamber 136.

The various sensors of the fatigue testing system 100 may be operablyconnected to a data acquisition (DAQ) device 141. An amplifier andcontrol system 140 may also be connected with the drive motor 105 toprovide output control of the same. In some embodiments, the DAQ device141 and amplifier/control system 140 may be mounted within the baseframe 157. An air source 131 connected in line with a pressure regulator134 may also be under control of the control system 140. Thesecomponents are, in turn, either directly or indirectly operablyconnected to a microprocessor-based computer 142. All systems may alsobe connected to an uninterrupted power supply (UPS) 143.

Software provided on the computer 142 can direct the system controller140 to provide full closed-loop control based on feedback measurementsreceived from the DAQ device 141 coupled with the sensors. In oneexample of this functionality, software may be configured to control thedriving amplitude and velocity of the drive motor 105 based upondifferential pressure transducer feedback. The system 100 may thereforecompensate for any changes during the high cycle testing cycle providingthe test samples 130 with the most optimal testing conditions. Anautomated test interface may be provided as part of the fatigue testingsystem 100 to run the test chamber 106 without direct management,ensuring proper testing conditions and safety mechanisms. The testinterface may be configured to provide closed-loop control based off anysystem measurement feedback.

An implementation of operation of the exemplary fatigue testing system100 of FIGS. 1-5B may be understood as follows. Initially, test samples130 need to be placed within the test chamber 106. In order to introducetest samples 130, the sample holders 129 must be accessed. Theadjustment nuts 146 on the top of the return chamber 136 may be rotatedto raise the adjustment posts 145 from their tightened positions withinthe distribution chamber manifold 153 and distribution chamber 126. Oncethe return chamber 136 and return chamber manifold 154 are raised, thesample holders 129 may be removed and replaced with alternate sampleholders or, as shown in this embodiment, samples positioned betweensample inlet to 147 and sample outlet to 148, may be removed andreplaced with new samples 130. The adjustment nuts 146 may then betightened to lower the return chamber 136 and return chamber manifold154 back into contact with the sample holders 129 to create fluid-tightflow path through each of the sample holders 129 between thedistribution chamber 126 and the return chamber 136. Note that when thereturn chamber 136 and return chamber manifold 154 are raised, thecenter conduit 124 telescopes upward with the upper conduit wall 124(2)sliding along the exterior of the lower conduit wall 124(1), thusmaintaining a fluid-tight connection within a center conduit 124.

In many applications, it may be desirable to monitor the pressuregradient across the sample holder 129. For such purposes, a conduit witha sensing device, for example, a pressure transducer, may be placed in aconduit that is attached at one end to a monitor outlet port 149 in thesample inlet tubes 147 and a monitor inlet port 150 on the sample outlettubes 148. In an alternative implementation, individual sensors (e.g.,pressure transducers) may be attached directly to each of the monitoroutlet ports 149 and monitor inlet ports 150 to measure absolutepressure at each of these locations. The absolute pressure data may becollected by the data acquisition component 141 and transferred to thecomputing device 142 for calculation of the pressure differential bytaking the difference between the absolute pressure values on either endof the sample holders 129.

Presuming that the correct size of cylinder 113 and corresponding driveadaptor 117 are already attached, the fatigue testing system 100 may becharged with the desired type and amount of working fluid through anyone of the inflow ports 143 in the plenum 118. The working fluid may bewater, saline, a saline/glycerin solution, a glycerin/water solution, ablood analog or substitute, or other type of fluid. In some embodiments,the working fluid may be selected to simulate one or more attributes ofhuman blood, such as density, viscosity or temperature. For example, incertain instances, physiological saline which does not simulate theviscosity of blood, but simulates density, may be used. In other casessaline/glycerin solution may be employed to simulate blood viscosity.

Depending upon the particular test samples 130 and desired testingconditions and dynamics, the fatigue system 100 may be filled with fluidup to an appropriate or desired level in the compliance chambers 135.Once fluid has been filled to a desired level, in some scenarios, thefatigue testing system 100 may be additionally pressurized with air oranother gas via an air source 131, e.g., an air compressor or sealedpressurized volume, connected to the air inlet 152 in the return chamber136 or via the compliance access ports 125 in the compliance chambers135. In some embodiments, a pressure regulator 134 may be interposedbetween the air source 131 and the air inlet 152 to regulate thepressure within the test chamber 106. Alternatively, the system 100 canbe pressurized by first sealing the system 100 and then changing theposition of the fluid driving piston 114 such that the available volumeis decreased.

The working fluid temperature is controlled via the fluid heaters 138and the temperature transducer 139. Upper and lower temperature boundsmay be set in the test software. At startup the system 100 will begin toheat until the upper bound is reached. As the input temperature fallsbelow the lower bound, the heaters 138 are again activated, thusmaintaining a mean temperature within acceptable bounds. This meantemperature is typically set to 37° C. to simulate physiologicconditions.

Once the system is filled with fluid, heated, and properly pressurized,testing operations under control of the computer 142 may be initiated.The computer 142 may control the operations of the linear drive motor105 to move the piston 114 and thus the diaphragm 115 up and down withinthe cylinder 113 and drive it after 117 to cyclically increase anddecrease the pressure within the test chamber 106. Under computercontrol and depending upon the size of the cylinder 113, the lineardrive motor 105 may cause the thrust rod 108 and the attached uppershaft extension 112 to move up and down thereby moving the cylinder 113and diaphragm 115 up and down to create a displacement of any size,small or large, to change the pressure within the test chamber 106. Therolling or eversion of the diaphragm 115 exerts very little resistanceto piston 14 movement. Thus, power requirements on the linear drivemotor 105 are small and current draw is minimized.

Fluid flow 110 through the test chamber 106 is indicated by the dashedlines initiating at the drive adaptor 117 traveling through the plenum118 and then through the sample holders 129 to exit through the returnconduit 168, the central return chamber 133, and the center conduit 124.Fluid flow can be prevented from moving upward in the center conduit 124by the one-way valve 127 housed within the distribution chamber manifold153. The conical diverter 122 helps direct fluid flow from the piston114 regularly and uniformly to each of the sample holders 129 about theperimeter of the test chamber 106.

The compliance chambers 135 provide excess volume area for fluid to moveinto when the piston 114 performs a compression stroke. As the pressureof the gas in the compliance chamber 135 increases, the volume occupiedby the gas decreases to provide additional volume for displacement ofthe liquid working fluid within the test chamber 106. The throttlevalves 132 restrict the rate of return flow of fluid within the testchamber 106 into the central return basin 133. The combination of thecompliance chambers 135 and the throttle valves 132 help controlundesirable pressure loading or pressure spikes within the sample holder129 and consequent adverse effects on the test samples 130 when thepiston 114 moves in a decompression stroke. The compliance chambers 135and the throttle valves 132 also generally help tune the test conditionsacross the sample holder 129. In addition to selecting the piston size,the displacement range, frequency, and waveform settings, the systemequilibrium pressure, and other settings, the compliance chambers 135and the throttle valves 132 may be adjusted as an additional aid infine-tuning appropriate differential pressure across the test samples130 in the sample holder 129.

As the piston 114 moves downward in decompression stroke, pressure onthe one-way valve 127 is released and fluid flow through the centerconduit 124 is initiated at a controlled rate depending upon theposition of the throttle valves 132. In this way, on the downwardstroke, the pressure in the test chamber 106 returns to an initial leveland excess volume returns to the compliance chambers 135. It should benoted that the compliance chambers 135 may be configured in a variety ofdifferent ways. In addition to merely containing a volume of air orother gas to act as an air spring, a membrane could be provided betweenthe liquid and the air or other gas or alternatively the space could befilled with a porous foam or other elastomeric material, therebyreducing or otherwise adjusting the spring factor provided by thecompliance chamber 135.

The dynamics of the system 100 can be controlled through the strokepattern, frequency, and volume of the piston 114, the flow setting ofthe throttle valves 132, and the spring force of the compliance chambers135. Additionally, the working fluid in the test chamber 106 may bepreset to a base or ambient pressure using a separate pressure source.For example, an air/gas source 131 (e.g., and air compressor or gastank) may be connected to and pressurize a fluid reservoir (not shown)in fluid contact with the working fluid in the test chamber 106. Apressure regulator 134 may be positioned between the air source 131 andthe introduction into the test chamber 106 for controlling the desiredsystem pressure. Alternatively, a pressurized gas may be introduceddirectly into the compliance chambers 135 through access ports 125, intothe central return basin 133 through inlet 152, or through any otherport (e.g., one of the sidewall ports 143 in the plenum 118) forcharging the test chamber to an appropriate base pressure for properfunction of the system 100 for the particular test configuration.

An alternate embodiment of a test chamber 206 for a fatigue testingsystem 200 is shown in FIG. 6. The test chamber 206 consists generallyof a lower chamber composed of a distribution chamber 226, a flowconditioning chamber 272, and a manifold 253, and an upper chambercomposed of a return chamber 236, a manifold 254 and a centralcompliance chamber 235. A fluid flow pathway 210 extends from the lowerchamber to the upper chamber which passes through a plurality of sampleholders 229 holding the device samples 230 being tested and a returnfluid flow pathway 228 extends from the upper chamber to the lowerchamber to return fluid from the upper chamber to the lower chamber inpreparation for a following cycle. A check valve 227 is inline with thereturn fluid flow pathway 228 between the upper and lower chambers toregulate back flow through the test chamber 206. A controllable centralthrottle valve 232 is also in-line with the upper and lower chambers toregulate the differential pressure across the test samples 230 duringoperation.

The fluid drive member, i.e., the piston 214, is provided within thecylinder 213 to pressurize and drive the working fluid upward throughthe drive adapter 217 and the plenum 218. The piston 214 is mounted onthe shaft extension 212 extending through the bearing 262 from thethrust rod of the linear motor (not shown). The flexible diaphragm ismounted on the piston 214 as in the prior embodiment and held in placeby a cap 216. The diaphragm 15 is preferably a cap-like memberconstructed of a non-reactive and flexible thin rubber, polymeric orsynthetic based material. The lateral surfaces of diaphragm 215 roll orevert as the piston 214 reciprocates within the cylinder 213 and driveadapter 217 between which the circumferential flange of the diaphragm issealed. These components are affixed to the support plenum 218 andmaintain the pressure seal along the circumference of the diaphragm 215.The plenum 218 serves as a mounting point for the drive system. Theplenum 218 may also include fluid ports for pressure monitoring andsystem draining.

During the primary or pressurization portion of a test cycle, the piston214 moves in a positive direction toward the test chamber 206 andcreates an initial pressurization and the working fluid flows up throughthe base plate 211 into the distribution chamber 226. The working fluidimpinges upon a generally conical flow baffle 222 and is directedradially outward to flow straighteners 271. The fluid is blocked fromentering the central return conduit 224 by a one-way exit valve 227. Theworking fluid then passes from the distribution chamber 226 and into aflow conditioning chamber 272, wherein it passes through a flowstraighteners 271, aligning the flow along the axis of the sampleholders 229. Sample adapters 247, 248 attach the sample holders 229 tothe distribution manifold 253 and the return manifold 255, allowing thesample holders 229 to be connected to the test chamber 206 in a leakfree manner.

A collateral pressure-sensing conduit 269 may be coupled to the sampleholder 229 and direct a fluid flow pathway toward a pressure transducer270 that serves to monitor the differential pressure gradient across thetest sample 330 within the sample holder 229. The differential pressuretransducers 270 monitor the pressure field across each test sample 230in the sample holders 229 during the testing cycle. Flexible or rigidtubing 269 is connected to the upper and lower sample adapters 247, 248,with the opposite ends of the conduit 269 being connected to thedifferential pressure transducer 270. Additional monitoring transducerscan be introduced through the sensor ports 221 and other ports builtinto the return chamber 236 as necessary.

Once the fluid flow has exited the sample holders 229 it flows into thereturn manifold 254 and return chamber 236. The working fluid passesaround a throttle valve 232 and a return fluid flow is communicatedthrough return conduit 224 to the lower chamber during a secondary ordepressurization portion of the test, thus completing the first portionof the test cycle. The throttle valve 232 is connected to the throttlevalve handle 232 a, which runs through a fluid-tight fitting on top ofcompliance chamber cap 221. The throttle valve handle 232 a isadjustable, such as by a threaded coupling to the cap 221 on thecompliance chamber 235. The throttle valve 232 can be adjusted up ordown, increasing or decreasing the resistance to fluid returning to thecentral return conduit 224 to aid in controlling the differentialpressure across the test sample 230. The amount of gas or the elasticityof the elastomeric material inside the compliance chamber 35 allows theuser to control the damping of the system and provides an additionaltool calibrate the ideal test conditions for the particular prostheticdevice. In the embodiment of FIG. 6, a single compliance chamber 235 isaxially centered over the test chamber 206 and provides a compliancevolume for all of the sample holders 230 in the test chamber 235.Similarly, in this embodiment only a single throttle valve 232 isprovided to control the return flow of the working fluid through thecenter return conduit 224.

FIG. 7 provides examples of three different pressure control signalsgenerating three different cyclical pressure waveforms 300 across aprosthetic device being tested. Because of the vertical orientation ofthe displacement components, the term “upstroke” is synonymous with apressurization stroke of the piston, i.e., that which exerts a positivepressure across the prosthetic device being tested and the term “downstroke” is synonymous with a depressurization stroke of the pump, i.e.,that which exerts a negative pressure across the prosthetic device beingtested. Typical test systems are only capable of driving the fluid witha regular sine wave 306. A non-regular pressure waveform may bedesirable for testing of certain devices as it allows the user tocontrol the rate of pressurization in the test system and optimize thetest conditions, while maintaining a desired operating frequency. Twoexemplary non-regular waveforms having short upstrokes 308 and downstrokes 310, respectively, are illustrated in FIG. 7. However, anyarbitrary pressure waveform may be generated and can be utilized todrive the motor and thereby the piston.

During the primary or pressurization portion 302 of the testing cycle,fluid is moved past the prosthesis within the housing tube. In anexemplary implementation in which valve prostheses are tested, thepositive upstroke forces the valve prosthesis to the open state. Duringa secondary or depressurization portion 304 of the test cycle, the flowis reversed and the valve prostheses 30 are closed. As the secondaryportion 304 of the cycle begins, the fluid moves through the throttlevalves into the central return conduit and back into the distributionchamber through the one-way valve. During return flow, the valveprosthesis remains closed due to the flow reversal and differentialpressure present between the distribution chamber and the returnchamber. The drive system returns to its starting position and theprocess is repeated, cycling the prosthetic devices. A single test cycleconsists of completion of both the primary portion 302 and secondaryportion 304 of the test cycle such that the prosthetic device passesthrough one open and-closed cycle.

The exemplary pressure waveforms 306, 308, 310 depicted in FIG. 7 may begenerated by the fatigue testing system or by other mechanicalmechanisms. It is envisioned that alternative mechanical orelectromechanical systems, such as those including gearing or cams todrive a pump, may be employed in a manner that generates the variablepressure waveforms as depicted in FIG. 7, in which the pressuregradients are variable over time.

As indicated above, the proposed technology may be integrated withsystem monitoring and controlling software. The computer software can beused to record and analyze data while controlling the dynamics of thesystem, as outlined in the exemplary process 400 shown FIG. 8. Data isfirst obtained by system hardware 402, i.e., the system sensors 406, andthen processed though data acquisition hardware 408 and transmitted tothe system computer for processing and formatting for presentation bythe system software 404.

All system inputs and outputs may be continuously monitored and directedinto software-based control and alarm system modules, allowing thesystem to automatically reconfigure or halt if any signal deviatesoutside of the user set bounds. The real-time data stream may beutilized for three primary purposes: data logging and graphing, alarmcondition indication, and test system control. The data logging andplotting subroutine 410 generates graphs and plots of pertinent signalswhile also creating a data file to allow for test documentation andoff-line analysis.

The alarm condition subroutine 412 analyzes data to determine if any ofthe test inputs or control signals have deviated outside of user definedalarm magnitudes. If an alarm is triggered, i.e. an alarm parametersexceeds its bounds as determined in operation 414, an alarm sequence isinitiated as indicated in operation 416. This sequence could trigger anumber of events. In one sequence, the software may halt the test systemin a specific manner; in another alarm sequence, the test software maynotify the operator; in yet another alarm sequence, the system may behalted and the user notified. As can be appreciated, there are a numberof actions that are available as part of an alarm sequence and thesesequence steps could be dependent on a number of parameters includingthe specific test samples and/or the specific test protocol.

The control loop subroutine 418 further monitors the control parametersbased on a user defined target input signals and/or parameters.Exemplary user set parameters may include pressure input parameters 428and motor drive waveform parameters 430. Each of these parameters may beset as static or variable. If the control loop determines that thereal-time sensor data received is outside the bounds of the userparameters as determined in operation 420, then the software adjusts theinput control parameters provided to the hardware control systems asindicated in operation 422. As one example, test system pressure may beset as the control parameter; therefore, the software will continuallyadjust the dynamics of the pressure regulator 424 to maintain thedesired pressure. In another example, the displacement of the driver maybe the control parameter; therefore, the software will continuallyadjust the system dynamics for the motor control 426 to maintain thedesired displacement. Again, as one can appreciate, the closed-loopcontrol of the test system can be applied to myriad parameters.

FIG. 9 illustrates an exemplary computer system 500 configured as partof the fatigue testing system as described herein. In oneimplementation, the computer system 500 typically includes at least oneprocessing unit 502 and memory 504. Depending upon the exactconfiguration and type of the computer system 500, the memory 504 may bevolatile (e.g., RAM), non-volatile (e.g., ROM and flash memory), or somecombination of both. The most basic configuration of the computer system500 need include only the processing unit 502 and the memory 504 asindicated by the dashed line 506.

The computer system 500 may further include additional devices formemory storage or retrieval. These devices may be removable storagedevices 508 or non-removable storage devices 510, for example, memorycards, magnetic disk drives, magnetic tape drives, and optical drivesfor memory storage and retrieval on magnetic and optical media. Storagemedia may include volatile and nonvolatile media, both removable andnon-removable, and may be provided in any of a number of configurations,for example, RAM, ROM, EEPROM, flash memory, CD-ROM, DVD, or otheroptical storage medium, magnetic cassettes, magnetic tape, magneticdisk, or other magnetic storage device, or any other memory technologyor medium that can be used to store data and can be accessed by theprocessing unit 502. Alarm monitoring, data acquisition, and closed loopcontrol software modules may be stored on the storage device forexecution by the processing unit 502 using any method or technology forstorage of data, for example, computer readable instructions, datastructures, and program modules.

The computer system 500 may also have one or more communicationinterfaces 512 that allow the system 500 to communicate with otherdevices. The communication interface 512 may be connected with anetwork. The network may be a local area network (LAN), a wide areanetwork (WAN), a telephony network, a cable network, an optical network,the Internet, a direct wired connection, a wireless network, e.g., radiofrequency, infrared, microwave, or acoustic, or other networks enablingthe transfer of data between devices. Data is generally transmitted toand from the communication interface 512 over the network via amodulated data signal, e.g., a carrier wave or other transport medium. Amodulated data signal is an electromagnetic signal with characteristicsthat can be set or changed in such a manner as to encode data within thesignal.

The computer system 500 may further have a variety of input devices 514and output devices 516. Exemplary input devices 514 may include sensors,a keyboard, a mouse, a tablet, and/or a touch screen device. Exemplaryoutput devices 516 may include a display and speakers. Such inputdevices 514 and output devices 516 may be integrated with the computersystem 500 or they may be connected to the computer system 500 via wiresor wirelessly, e.g., via IEEE 802.11 or Bluetooth protocol. Theseintegrated or peripheral input and output devices are generally wellknown and are not further discussed herein. Other functions, forexample, handling network communication transactions, may be performedby an operating system in the nonvolatile memory 504 of the computersystem 500.

The technology described herein may be implemented as logical operationsand/or modules in one or more systems. The logical operations may beimplemented as a sequence of processor-implemented steps executing inone or more computer systems and as interconnected machine or circuitmodules within one or more computer systems. Likewise, the descriptionsof various component modules may be provided in terms of operationsexecuted or effected by the modules. The resulting implementation is amatter of choice, dependent on the performance requirements of theunderlying system implementing the described technology. Accordingly,the logical operations making up the embodiments of the technologydescribed herein are referred to variously as operations, steps,objects, or modules. Furthermore, it should be understood that logicaloperations may be performed in any order, unless explicitly claimedotherwise or a specific order is inherently necessitated by the claimlanguage.

In some implementations, articles of manufacture are provided ascomputer program products that cause the instantiation of operations ona computer system to implement the invention. One implementation of acomputer program product provides a computer program storage mediumreadable by a computer system and encoding a computer program. It shouldfurther be understood that the described technology may be employed inspecial purpose devices independent of a personal computer.

All directional references (e.g., proximal, distal, upper, lower,upward, downward, left, right, lateral, longitudinal, front, back, top,bottom, above, below, vertical, horizontal, radial, axial, clockwise,and counterclockwise) are only used for identification purposes to aidthe reader's understanding of the present invention, and do not createlimitations, particularly as to the position, orientation, or use of theinvention. Connection references (e.g., attached, coupled, connected,and joined) are to be construed broadly and may include intermediatemembers between a collection of elements and relative movement betweenelements unless otherwise indicated. As such, connection references donot necessarily infer that two elements are directly connected and infixed relation to each other. The exemplary drawings are for purposes ofillustration only and the dimensions, positions, order and relativesizes reflected in the drawings attached hereto may vary.

The above specification, examples and data provide a completedescription of the structure and use of exemplary embodiments of theinvention. Although various embodiments of the invention have beendescribed above with a certain degree of particularity, or withreference to one or more individual embodiments, those skilled in theart could make numerous alterations to the disclosed embodiments withoutdeparting from the spirit or scope of this invention. Other embodimentsare therefore contemplated. It is intended that all matter contained inthe above description and shown in the accompanying drawings shall beinterpreted as illustrative only of particular embodiments and notlimiting. Changes in detail or structure may be made without departingfrom the basic elements of the invention as defined in the followingclaims.

What is claimed is:
 1. A method for operating an accelerated cyclic testsystem for evaluating a valved prosthetic device comprising driving atest system fluid cyclically above a normal physiological rate, at anaccelerated pulsed rate of greater than 200 beats per minute within thetest system; storing a volume of test system fluid in an excess volumearea during a system driving stroke that opens the valved prostheticdevice; and releasing the stored volume of test system fluid during areturn stroke that closes the valved prosthetic device.
 2. The method ofclaim 1, wherein the excess volume area enlarges in response to apressure on the test system fluid during the driving stroke anddecreases during the return stroke.
 3. The method of claim 2, whereinthe excess volume area provides a spring force counter to and inresponse to the pressure on the test system fluid.
 4. The method ofclaim 3 further comprising altering a spring factor of the spring forceprovided by the excess volume area through selection of a materialforming at least a portion of a boundary of the excess volume area. 5.The method of claim 4, wherein the material is an elastomeric materialthat expands and contracts in response to the pressure on the testsystem.
 6. The method of claim 1, further comprising compressing avolume of a compressible gas with the volume of test system fluid toprovide a spring force counter to and in response to a pressure on thetest system fluid when the volume of test system fluid is stored in theexcess volume area.
 7. The method of claim 6 further comprising alteringa spring factor of the spring force provided by the excess volume areaby adjusting the volume of the compressible gas.